1. Field of the Invention
The present invention relates to a semiconductor switching device, and particularly to a large current capacity switching device having a reduced switching loss.
2. Description of the Prior Art
Semiconductor switching devices are extensively used in power supplies for microwave ovens, induction heating power supplies for electromagnetic cooking apparatus, uninterrupted power supplies, or the like. These semiconductor switching devices typically use semiconductor switching elements such as bipolar transistors (BJTs) and insulated gate bipolar junction transistors (IGBTs) because of their large current capacity and low saturation voltages. These semiconductor elements are controlled by driving circuits connected to gates of the IGBTs or bases of the transistors to turn on and off electric current at predetermined frequencies.
To reduce power consumption of these switching devices using the BJTs or IGBTs, it is of great importance to reduce switching loss of the semiconductor switching elements.
FIG. 1 is a diagram illustrating a switching characteristic of a switching device for an induction heating power supply in which an IGBT is driven by a driving circuit. In FIG. 1, Ic is a collector current of the IGBT, VCE is a collector-to-emitter voltage of the IGBT. During the on-interval during which a positive potential is applied to the gate of the IGBT, a large amount of current flows through the IGBT, and the on-state voltage (called the saturation voltage hereinafter) VCE(sat) is low. In contrast, during the off-interval in which a negative potential is applied to the gate, the current is interrupted. Thus, in FIG. 1, the IGBT is periodically driven with the on-interval of approximately 30 .mu.s, and the off-interval of approximately 20 .mu.s. Here, the consumption power involved in the switching of the IGBT, namely, the switching loss of the IGBT, corresponds to shaded areas of FIG. 1: The power consumption corresponding to a shaded area A is a turn-on loss determined by the saturation voltage VCE(sat) and the power consumption corresponding to a shaded area B is a turn-off loss determined by a tail portion of the current waveform of the IGBT. Currently, the turn-off loss B accounts for a larger percentage of the switching loss, and hence, reduction of the turn-off loss B brings about the improvement of the power efficiency. The turn-off loss B can be reduced by shortening the fall time tf of the IGBT.
The shortening of the fall time tf of minority carrier devices such as IGBTs and BJTs, however, generally increases the saturation voltage VCE(sat). In other words, there is a trade-off between the fall time tf and the saturation voltage VCE(sat). Therefore, there is also a trade-off between the turn-off loss and the turn on-loss, and hence, the reduction in the switching loss is restricted within a certain limit by inherent characteristics of the device elements.
One of the conventional techniques to decrease the turn-off loss with restricting the turn-on loss is shown in FIG. 2. In FIG. 2, a BJT (Bipolar Junction Transistor) 1 is connected in parallel with a high speed MOSFET 2. The BJT 1 is driven by a drive circuit 10 and the MOSFET 2 is driven by a drive circuit 20 so that an input terminal 3 is connected with or disconnected from an output terminal 4. When the device is turned off in this arrangement, the base current IB of the BJT 1 is cut off prior to the gate voltage VGS of the MOSFET 2, as shown in FIG. 3, so that the collector current I1 of the BJT 1 drops previous to the drain current I2 of the MOSFET 2. Thus, the load current IL of the device is interrupted at a high speed corresponding to the fall time tfMOS of the MOSFET 2, which is shorter than the fall time tfBJT of the BJT 1.
The BJT 1 and the MOSFET 2, however, are basically different drive mode devices: a current drive device and a voltage control device, respectively. For this reason, a single drive circuit is not enough to establish necessary cutoff timing between the BJT 1 and the MOSFET 2: two drive circuits 10 and 20 are needed, thereby complicating the circuit and increasing the occupied area of the device.